Introduction

Predation is used here to include all
"+/-" interactions in which one organism consumes all or part of another.
This includes predator-prey, herbivore-plant, and parasite-host interactions.
These linkages are the prime movers of energy through food chains. They
are an important factor in the ecology of populations, determining mortality
of prey and birth of new predators.
Predation is an important evolutionary force: natural selection favors
more effective predators and more evasive prey. "Arms races" have been
recorded in some snails, which over time become more heavily armored prey,
and their predators, crabs, which over time develop more massive claws
with greater crushing power. Predation is widespread and easy to observe.
Neither its existence nor its importance is in doubt.

The Development of Predation Theory

Mathematical models of predation are
amongst the oldest in ecology. The Italian mathematician Volterra is said
to have developed his ideas about predation from watching the rise and
fall of Adriatic fishing fleets. When fishing was good, the number of fishermen
increased, drawn by the success of others. After a time, the fish declined,
perhaps due to over-harvest, and then the number of fishermen also declined.
After some time, the cycle repeated.

Linx chasing Hare

The idea that a coupled system of
predator and prey would cycle gained further support from analyses of fur
trapping records of the Hudson's Bay Company. The number of furs purchased
at the Company's forts was meticulously recorded, for well over 100 years.
An analysis of the numbers of snowshoe hares, and one of their main predators,
the lynx, provides a remarkable record of a predator-prey
cycle. Peaks and valleys can be easily observed at roughly 8-10
year intervals.

Logic and mathematical theory
suggest that when prey are numerous their predators increase in numbers,
reducing the prey population, which in turn causes predator number to decline.
The prey population eventually recovers, starting a new cycle.

T

Paramecium, which also proved useful
in test-tube studies of competition, was placed in culture with a predaceous
protozoan. These laboratory studies found that cycles were short-lived,
and the system soon collapsed. However, if one added more paramecium every
few days, the expected cycle was observed.

These results suggested that the
predator-prey system was inherently self-annihilating without some outside
immigration. The question then arose: why are predator-prey cycles in nature
apparently stable, while laboratory cultures quickly collapse?

What Stabilizes Predator-Prey Systems
in Nature?

Observing that frequent additions of
paramecium produced predator-prey cycles in a test-tube led to the idea
that in a physically heterogeneous world, there would always be some pockets
of prey that predators happened not to find and eliminate. Perhaps when
the predator population declined, having largely run out of prey, these
remaining few could set off a prey rebound. Spatial heterogeneity in the
environment might have a stabilizing effect.

A laboratory experiment using a complex
laboratory system supports this explanation. A predaceous mite feeds on
an herbivorous mite, which feeds on oranges. A complex laboratory system
completed four classic cycles, before collapsing.

Observations of prickly pear cactus
and the cactus moth in Australia support this lab experiment. This South
American cactus became a widespread nuisance in Australia, making large
areas of farmland unusable. When the moth, which feeds on this cactus,
was introduced, it rapidly brought the cactus under control. Some years
later both moth and cactus were rare, and it is unlikely that the casual
observer would ever think that the moth had accomplished this. Once the
cactus became sufficiently rare, the moths were also rare, and unable to
find and eliminate every last plant. Inadequate dispersal is perhaps the
only factor that keeps the cactus moth from completely exterminating its
principal food source, the prickly pear cactus.

Prey defenses can be a stabilizing
factor in predator-prey interactions. Predation can be a strong agent of
natural selection. Easily captured prey are eliminated, and prey with effective
defenses (that are inherited) rapidly dominate the population. Examples
include camouflage in the peppered moth, and prey that are nocturnal to
escape detection. Bats capture moths in flight, using sonar to detect them;
some moths are able to detect incoming sonar, and take evasive action.
Perhaps seriously unbalanced system simply disappear, and those that persist
are ones in which the predator is not "too effective", likely because the
prey has adaptations to reduce its vulnerability.

The availability of a second prey
type -- an alternate prey -- can be stabilizing or destabilizing. Often
a predator eats more than one prey. If a predator switches between prey
A and B on the basis of their frequency, it will eat A when B is rare and
B when A is rare. The prey should exhibit mild oscillations, and the predator
should fluctuate little. This would stabilize prey abundances. However,
if one prey species is abundant and the predator is unable to reduce its
numbers, the result might be the maintenance of a continuously high predator
density. Such an abundant predator might then eliminate a second prey species.
This is a destabilizing effect of an alternative prey. The hare-caribou-lynx
relationship in Newfoundland is a complex example of such a destabilizing
effect.

Complex Interactions in Ecological Communities

Predation can have far-reaching effects
on biological communities. A starfish is the top predator upon a community
of invertebrates inhabiting tidally inundated rock faces in the Pacific
Northwest. The rest of the community included mollusks, barnacles and other
invertebrates, for a total of 12 species (not counting microscopic taxa).
The investigator removed the starfish by hand, which of course reduced
the number of species to 11. Soon, an acorn barnacle and a mussel began
to occupy virtually all available space, out competing other species. Species
diversity dropped from more than 12 species to essentially 2. The starfish
was a keystone
predator, keeping the strongest competitors in check. Although it was
a predator, it helped to maintain a greater number of species in the community.
Its beneficial impact on species that were weak competitors is an example
of an indirecteffect.

When non-native species (exotics)
invade an area, they often create "domino" effects, causing many other
species to increase or decrease. The rainbow trout, beautiful, tasty, and
beloved by anglers, has been purposefully spread to virtually all parts
of the world where it can survive. In New Zealand, it has out-competed
the native fishes, which now are found only above waterfalls that act as
barriers to trout dispersal. Because it is a more effective predator than
the native fish species, the invertebrates that are prey to the trout are
reduced in abundance wherever trout occur. Algae, which are grazed by the
invertebrates, increase because of reduced grazing pressure. This is an
example of a trophiccascade.

Introduction of the opossum shrimp
to Flathead Lake, Montana, is yet another example of complex interactions
in ecological communities.

Summary

Predation, a "+/-" interaction, includes
predator-prey, herbivore-plant, and parasite-host interactions. These linkages
are the prime movers of energy through food chains and are an important
factor in the ecology of populations, determining mortality of prey and
birth of new predators. Mathematical models and logic suggests that a coupled
system of predator and prey should cycle: predators increase when prey
are abundant, prey are driven to low numbers by predation, the predators
decline, and the prey recover, ad infinitum. Some simple systems do cycle,
particularly those of the boreal forest and tundra, although this no longer
seems the rule. In complex systems, alternative prey and multi-way interactions
probably dampen simple predator-prey cycles.

Predator-prey systems are potentially
unstable, as is seen in the lab where predators often extinguish their
prey, and then starve. In nature, at least three factors are likely to
promote stability and coexistence. Due to spatial heterogeneity in the
environment, some prey are likely to persist in local "pockets" where they
escape detection. Once predators decline, they prey can fuel a new round
of population increase. Prey evolve behaviors, armor, and other defenses
that reduce their vulnerability to predators. Alternative prey may provide
a kind of refuge, because once a prey population becomes rare, predators
may learn to search for a different prey species.

Predation, while not the only cause
of complex community interactions, has often been shown to have strong
indirect effects and cascading effects. Predation also can be a strong
agent of natural selection, as we saw in the case of the peppered
moth.